fix(bun-spawn-shim): eliminate globalThis.Bun top-level destructures for Electron/Node compat

Root cause: bun build --target bun inlines top-level
  var { spawn } = globalThis.Bun;
for every file that contains 'import { spawn } from "bun"'. On Node/Electron
where globalThis.Bun is undefined, this crashes with
  Cannot destructure property 'spawn' of 'globalThis.Bun' as it is undefined.
26 source files had this import; the bundled output had 25 top-level destructures.

Fix:
- Add src/shared/bun-spawn-shim.ts: a thin wrapper that
  - delegates to Bun.spawn/spawnSync when globalThis.Bun is present (real Bun)
  - falls back to static ESM imports of node:child_process otherwise
  - uses static 'import { spawn } from "node:child_process"' so Bun bundler
    does NOT emit any globalThis.Bun destructures for this module
- Replace all 26 'from "bun"' spawn/spawnSync imports with relative paths to shim
- Replace 4 direct Bun.spawn() call sites with shim's spawn()
- Remove src/electron-compat.ts and script/prepend-electron-shim.ts (no longer needed)
- Update src/electron-compat.test.ts to assert 0 top-level globalThis.Bun destructures

Verification: grep -c '} = globalThis.Bun;' dist/index.js → 0 (was 25)
All 5921 tests pass (1 pre-existing timeout failure unrelated to this change).

Fixes #3797

src/tools/interactive-bash/tmux-path-resolver.ts

@@ -1,4 +1,4 @@
-import { spawn } from "bun"
+import { spawn } from "../../shared/bun-spawn-shim"
 
 let tmuxPath: string | null = null
 let initPromise: Promise<string | null> | null = null